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Transgressive Systems Tract (transgressive + system_tract)
Selected AbstractsSequence stratigraphy of the upper Millstone Grit (Yeadonian, Namurian), North WalesGEOLOGICAL JOURNAL, Issue 5 2007Rhodri M. Jerrett Abstract The upper Millstone Grit strata (Yeadonian, Namurian) of North Wales have been studied using sedimentological facies analysis and sequence stratigraphy. These strata comprise two cyclothems, each containing prodelta shales (Holywell Shale) that pass gradationally upwards into delta-front and delta-plain deposits (Gwespyr Sandstone Formation). The deltas formed in shallow water (<100,m), were fluvial-dominated, had elongate and/or sheet geometries and are assigned to highstand systems tracts. Two delta complexes with distinctive sandstone petrographies are identified: (1) a southerly derived, quartzose delta complex sourced locally from the Wales-Brabant Massif, and (2) a feldspathic delta complex fed by a regional source(s) to the north and/or west. The feldspathic delta complex extended further south in the younger cyclothem. A multistorey braided-fluvial complex (Aqueduct Grit, c. 25,m thick) is assigned to a lowstand systems tract, and occupies an incised valley that was eroded into the highstand feldspathic delta complex in the younger cyclothem. A candidate incised valley cut into the highstand feldspathic delta complex in the older cyclothem is also tentatively identified. Transgressive systems tracts are thin (<5,m) and contain condensed fossiliferous shales (marine bands). The high-resolution sequence stratigraphic framework interpreted for North Wales can be readily traced northwards into the Central Province Basin (,Pennine Basin'), supporting the notion that high-frequency, high-magnitude sea-level changes were the dominant control on stratigraphic architecture. Copyright © 2007 John Wiley & Sons, Ltd. [source] Depositional environment and sequence architecture of the Silurian Coralliferous Group, Southern Pembrokeshire, UKGEOLOGICAL JOURNAL, Issue 3 2002Robert D. Hillier Abstract The Lower Silurian siliciclastic Coralliferous Group is shown to have been deposited in an intra-shelf position 10,15,km south of the palaeogeographic shelf-break of the Welsh Basin. After a phase of thermal subsidence related to the development of the predominantly Llandovery Skomer Volcanic Group, the shelf basin was transgressed. This transgression was punctuated by an episode of tectonic uplift in southern Pembrokeshire, resulting in subaerial exposure of the shelf and a significant basinward shift in sedimentary environments. Erosion and sediment bypass ensued, with coarse-grained low-sinuosity fluvial channels transporting sediment to the northerly Welsh Basin, where significant submarine fans developed. During the early Telychian, renewed transgression took place, with lowstand gravels being ravined and reworked into parasequences of the transgressive systems tract. These thin, coarse-grained parasequences record deposition within high-energy wave-dominated shoreface/inner shelf environments. Further coastal onlap resulted in the closing down of significant coarse-grained sediment supply, with the remaining Coralliferous Group being dominated by wave-influenced silts, mud-shales and thin sandstones comprising the highstand systems tract. Copyright © 2002 John Wiley & Sons, Ltd. [source] Fluvial response to sea-level changes: a quantitative analogue, experimental approachBASIN RESEARCH, Issue 3 2001M. W. I. M. Van Heijst ABSTRACT Quantitative evaluation of fluvial response to allogenic controls is crucial for further progress in understanding the stratigraphic record in terms of processes that control landscape evolution. For instance, without quantitative insight into time lags that are known to exist between sea-level change and fluvial response, there is no way to relate fluvial stratigraphy to the sea-level curve. It is difficult to put firm constraints on these time-lag relationships on the basis of empirical studies. Therefore, we have started to quantify time-averaged erosion and deposition in the fluvial and offshore realms in response to sea-level change by means of analogue modelling in a 4 × 8-m flume tank. The rate of sea-level change was chosen as an independent variable, with other factors such as sediment supply, discharge and initial geometry kept constant over the course of 18 experiments. Our experimental results support the common view that neither fall nor rise in sea level affects the upstream fluvial system instantaneously. An important cause for the delayed fluvial response is that a certain amount of time is required to connect initial incisions on the newly emergent shelf (canyons) with the fluvial valley. Lowering of the fluvial longitudinal profile starts only after the connection of an active shelf canyon with the fluvial valley; until that moment the profile remains steady. We quantified the process of connection and introduced the quantity ,connection rate'. It controlled, in conjunction with the rate of sea-level fall: (1) the amount of fluvial degradation during sea-level fall; (2) the total sediment volume that bypasses the shelf edge; (3) the percentage of fluvial relative to shelf sediment in the lowstand delta; (4) the volume of the transgressive systems tract and (5) the amount of diachroneity along the sequence boundary. Our experiments demonstrate also that the sequence-stratigraphic concept is difficult to apply to continental successions, even when these successions have been deposited within the influence of sea level. [source] The Tendaguru Formation (Late Jurassic to Early Cretaceous, southern Tanzania): definition, palaeoenvironments, and sequence stratigraphyFOSSIL RECORD-MITTEILUNGEN AUS DEM MUSEUM FUER NATURKUNDE, Issue 2 2009Robert Bussert Abstract The well-known Late Jurassic to Early Cretaceous Tendaguru Beds of southern Tanzania have yielded fossil plant remains, invertebrates and vertebrates, notably dinosaurs, of exceptional scientific importance. Based on data of the German-Tanzanian Tendaguru Expedition 2000 and previous studies, and in accordance with the international stratigraphic guide, we raise the Tendaguru Beds to formational rank and recognise six members (from bottom to top): Lower Dinosaur Member, Nerinella Member, Middle Dinosaur Member, Indotrigonia africana Member, Upper Dinosaur Member, and Rutitrigonia bornhardti-schwarzi Member. We characterise and discuss each member in detail in terms of derivation of name, definition of a type section, distribution, thickness, lithofacies, boundaries, palaeontology, and age. The age of the whole formation apparently ranges at least from the middle Oxfordian to the Valanginian through Hauterivian or possibly Aptian. The Tendaguru Formation constitutes a cyclic sedimentary succession, consisting of three marginal marine, sandstone-dominated depositional units and three predominantly coastal to tidal plain, fine-grained depositional units with dinosaur remains. It represents four third-order sequences, which are composed of transgressive and highstand systems tracts. Sequence boundaries are represented by transgressive ravinement surfaces and maximum flooding surfaces. In a more simple way, the depositional sequences can be subdivided into transgressive and regressive sequences/systems tracts. Whereas the transgressive systems tracts are mainly represented by shallow marine shoreface, tidal channel and sand bar sandstones, the regressive systems tracts predominantly consist of shallow tidal channel, tidal flat, and marginal lagoonal to supratidal deposits. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||